fix: Improve symlink check and sysconfig path handling

docs/changelog/2786.bugfix.rst

@@ -0,0 +1,3 @@
+Creating a virtual environment on a filesystem without symlink-support would fail even with `--copies`
+Make `fs_supports_symlink` perform a real symlink creation check on all platforms.
+Contributed by :user:`esafak`.

src/virtualenv/create/via_global_ref/api.py

@@ -62,7 +62,12 @@ def add_parser_arguments(cls, parser, interpreter, meta, app_data):
             help="give the virtual environment access to the system site-packages dir",
         )
         if not meta.can_symlink and not meta.can_copy:
-            msg = "neither symlink or copy method supported"
+            errors = []
+            if meta.symlink_error:
+                errors.append(f"symlink: {meta.symlink_error}")
+            if meta.copy_error:
+                errors.append(f"copy: {meta.copy_error}")
+            msg = f"neither symlink or copy method supported: {', '.join(errors)}"
             raise RuntimeError(msg)
         group = parser.add_mutually_exclusive_group()
         if meta.can_symlink:

src/virtualenv/discovery/py_info.py

@@ -219,7 +219,9 @@ def is_venv(self):
         return self.base_prefix is not None
 
     def sysconfig_path(self, key, config_var=None, sep=os.sep):
-        pattern = self.sysconfig_paths[key]
+        pattern = self.sysconfig_paths.get(key)
+        if pattern is None:
+            return ""
         if config_var is None:
             config_var = self.sysconfig_vars
         else:

src/virtualenv/info.py

@@ -34,18 +34,20 @@ def fs_supports_symlink():
     if _CAN_SYMLINK is None:
         can = False
         if hasattr(os, "symlink"):
-            if IS_WIN:
-                with tempfile.NamedTemporaryFile(prefix="TmP") as tmp_file:
-                    temp_dir = os.path.dirname(tmp_file.name)
-                    dest = os.path.join(temp_dir, f"{tmp_file.name}-{'b'}")
-                    try:
-                        os.symlink(tmp_file.name, dest)
-                        can = True
-                    except (OSError, NotImplementedError):
-                        pass
-                LOGGER.debug("symlink on filesystem does%s work", "" if can else " not")
-            else:
-                can = True
+            # Creating a symlink can fail for a variety of reasons, indicating that the filesystem does not support it.
+            # E.g. on Linux with a VFAT partition mounted.
+            with tempfile.NamedTemporaryFile(prefix="TmP") as tmp_file:
+                temp_dir = os.path.dirname(tmp_file.name)
+                dest = os.path.join(temp_dir, f"{tmp_file.name}-{'b'}")
+                try:
+                    os.symlink(tmp_file.name, dest)
+                    can = True
+                except (OSError, NotImplementedError):
+                    pass  # symlink is not supported
+                finally:
+                    if os.path.lexists(dest):
+                        os.remove(dest)
+            LOGGER.debug("symlink on filesystem does%s work", "" if can else " not")
         _CAN_SYMLINK = can
     return _CAN_SYMLINK
 

tests/unit/create/test_creator.py

@@ -25,6 +25,7 @@
 from virtualenv.__main__ import run, run_with_catch
 from virtualenv.create.creator import DEBUG_SCRIPT, Creator, get_env_debug_info
 from virtualenv.create.pyenv_cfg import PyEnvCfg
+from virtualenv.create.via_global_ref import api
 from virtualenv.create.via_global_ref.builtin.cpython.common import is_macos_brew
 from virtualenv.create.via_global_ref.builtin.cpython.cpython3 import CPython3Posix
 from virtualenv.discovery.py_info import PythonInfo
@@ -690,3 +691,63 @@ def func(self):
 
     cmd = [str(tmp_path), "--seeder", "app-data", "--without-pip", "--creator", "venv"]
     cli_run(cmd)
+
+
+def test_fallback_to_copies_if_symlink_unsupported(tmp_path, python, mocker):
+    """Test that creating a virtual environment falls back to copies when filesystem has no symlink support."""
+    if is_macos_brew(PythonInfo.from_exe(python)):
+        pytest.skip("brew python on darwin may not support copies, which is tested separately")
+
+    # Given a filesystem that does not support symlinks
+    mocker.patch("virtualenv.create.via_global_ref.api.fs_supports_symlink", return_value=False)
+
+    # When creating a virtual environment (no method specified)
+    cmd = [
+        "-v",
+        "-p",
+        str(python),
+        str(tmp_path),
+        "--without-pip",
+        "--activators",
+        "",
+    ]
+    result = cli_run(cmd)
+
+    # Then the creation should succeed and the creator should report it used copies
+    assert result.creator is not None
+    assert result.creator.symlinks is False
+
+
+def test_fail_gracefully_if_no_method_supported(tmp_path, python, mocker):
+    """Test that virtualenv fails gracefully when no creation method is supported."""
+    # Given a filesystem that does not support symlinks
+    mocker.patch("virtualenv.create.via_global_ref.api.fs_supports_symlink", return_value=False)
+
+    # And a creator that does not support copying
+    if not is_macos_brew(PythonInfo.from_exe(python)):
+        original_init = api.ViaGlobalRefMeta.__init__
+
+        def new_init(self, *args, **kwargs):
+            original_init(self, *args, **kwargs)
+            self.copy_error = "copying is not supported"
+
+        mocker.patch("virtualenv.create.via_global_ref.api.ViaGlobalRefMeta.__init__", new=new_init)
+
+    # When creating a virtual environment
+    with pytest.raises(RuntimeError) as excinfo:
+        cli_run(
+            [
+                "-p",
+                str(python),
+                str(tmp_path),
+                "--without-pip",
+            ],
+        )
+
+    # Then a RuntimeError should be raised with a detailed message
+    assert "neither symlink or copy method supported" in str(excinfo.value)
+    assert "symlink: the filesystem does not supports symlink" in str(excinfo.value)
+    if is_macos_brew(PythonInfo.from_exe(python)):
+        assert "copy: Brew disables copy creation" in str(excinfo.value)
+    else:
+        assert "copy: copying is not supported" in str(excinfo.value)